The internal combustion engine, fueled by liquid petroleum products, has long been the mainstay for supplying mechanical power to a broad variety of mobile and stationary machines. There have been many developments for improving the fuel conversion efficiency of such engines. Although most internal combustion engines which are commercially produced and used today utilize reciprocating pistons which are confined to the motion limits permitted by a crank shaft and interconnected connecting rods, there have also been free piston internal combustion engines. For example, free piston internal combustion engines are shown in U.S. Pat. Nos. 4,873,822; 5,123,245; 5,363,651; 4,530,317; 4,415,313; and 4,205,528. Although most of these engines can be designed and operated to provide a high efficiency at a single selected power output load condition, few engines are called upon to operate under only one load condition. Most internal combustion engines must supply power which varies over a broad range from a low power to a high power.
Three parameters which are important to both the efficiency and the power of an internal combustion engine are stroke or displacement, expansion ratio, and compression ratio. Conventional crank-type internal combustion engines permit no controlled adjustment of any of these parameters. The efficiency of an internal combustion engine is also a function of the ratio of the compression ratio to the expansion ratio. In the conventional internal combustion engine, neither is variable. The power of an internal combustion engine is proportional to the mass flow of air, properly mixed with fuel, through the combustion chamber and therefore is also a function of piston displacement. However, piston displacement is not variable in a crank-type engine.
It is a feature and object of the present invention to provide a free piston internal combustion engine in which not only are all four of these parameters controllably variable, but additionally the expansion ratio and the compression ratio are adjustable independently of each other. This permits the engine to operate with a different expansion ratio than compression ratio and also allows the displacement or stroke of the engine to be controlled. Consequently, upon a low power demand the engine of the present invention can operate with an expansion ratio which is considerably greater than the compression ratio so that it can operate with more nearly full expansion, resulting in a higher proportion of the heat energy of combustion being converted to mechanical output power. For greater power demands, both the engine displacement and the expansion ratio can be varied so as to achieve maximum efficiency for a given power demand.